The knee also has a lubrication system, the cartilage and meniscus. These are white solids that your body makes to keep your joints friction free. Unfortunately, only the very outer edge of the meniscus and cartilage are alive. They grow slowly to fill in the parts that have been gradually worn away. However, if you injure the outer parts, they stop growing and it's all over for you, it's just a matter of time.
The cartilage can be torn, broken, or worn away. If it's worn away, you will have knee pain for the rest of your life. Eventually your body will lay down calcium to try to substitute for the cartilage. When the cartilage is severely damaged and you have bone-bone contact, you have Osteoarthritis.
If the cartilage is torn, then there can be a loose piece or a flap that moves around. These can cause your knee to "lock up," or at the minimum cause a fair amount of pain. These injuries will be "scoped." This means they make a cut into your knee about 1/2" wide, then push in with a little tiny tube with a light and a camera and look around. When they find the problem, they push in another small tube with tiny cutters and pinchers and pull the offending piece out. Then they pull out the tubes, give you a Snoopy band-aid, and loan you some crutches for about a week. A couple weeks later you're on the field with minor pain, and a couple months later you're back to normal.
Finally, you can have a tear of the meniscus. These are wedge-shaped pieces of cartilage that help with knee stability and low friction. If you have a small tear, they might scope it and "clean it up" a bit. If you have torn your meniscus in two, it's likely they will cut into your knee and remove part or all the meniscus. You can then play with a bit of pain, and are looking forward to more serious pain when / if you hit 50.